Number of Good Binary Strings (Dynamic Programming LeetCode 2533)

In the “Example Walkthrough” section, following line is not clear.

"We also check for zero blocks, but 5 - zeroGroup is less than 0 and thus, cannot form a valid string. "

This explanation does not make sense. as zeroGroup is 3 and 5 - 3 = 2 which is not less than 0